1. Field of the Invention
This invention relates to workpiece holders for presenting a plurality of workpieces to an operation involved in their manufacture. More particularly, it relates to such a workpiece holder especially designed for operations in the assembly of semiconductor device packages. Most especially, it relates to such a workpiece holder designed for semiconductor device package assembly operations carried out on a continuous feed, infrared (IR) and convection heated ovens.
Semiconductor packaging assembly operations are typically characterized by several steps involving heating a part in order to coat it or to bond it to another part. For example, a lead frame may be bonded to a base which will have an integrated circuit chip mounted in it. A cover may then be bonded to the lead frame by melting a glass layer on it. In some cases, glass frits may be applied to one or more of the parts in separate steps prior to bonding. In high volume manufacturing environments for the assembly of such packages, it is a common practice to employ continuous ovens through which the parts are fed on some type of workpiece holder. The parts are typically heated by application of IR energy or convection heat in the ovens. If a bonding operation is being carried out in the oven, it often involves reflow bonding of solder or the heat cure of a potting or casting compound.
In such a continuous oven, workpiece holders for the parts must be capable of supporting the parts in a position to receive IR energy efficiently. They must be built to reasonably tight dimensional tolerances in order to pass through the ovens reliably and support the parts in a precisely predetermined relationship in which they are not free to move before the bonding operation is completed. They must not warp on heating to the temperatures employed in the assembly operations. It would be highly desirable if they could have the capability of holding several different types of standard semiconductor package parts for use with different products employing different parts in their packages.
Depending on the throughput capacity desired, dimensions of semiconductor package assembly equipment of similar design may vary. Consequently, a workpiece holder for semiconductor device package piece parts which is made of easily assembled modular parts would allow dimensional flexibility for use with assembly machines of different dimensions but otherwise similar construction. This also means that workpiece supports of the workpiece holder with overall standard dimensions can be substituted for one another in the holder to give an increased flexibility for using the holder with different types of parts.
2. Description of the Prior Art
Typical prior art workpiece holders used in the above described environment are essentially flat plates of sheet metal with apertures formed to receive the parts in them. Such a holder is inexpensive to fabricate, but not particularly suited for an IR environment due to its large surface area normal to the IR beams in an IR heated oven. Holders presenting a smaller surface area to the IR beams have been fabricated from metal strips by brazing them together, but such fabrication methods are expensive and time consuming. Such prior art workpiece holders have also been limited to holding at most only two particular parts, with a completely different holder design required for each different set of parts to be mounted on the holder. Each different package design requires a different workpiece holder design. This means more complexity is introduced into high volume automated semiconductor package assembly operations.
Considerable effort has been devoted to developing a workpiece holder suited to the demands of the semiconductor package assembly environment. However, a need remains for a workpiece holder ideally suited for semiconductor package assembly operations.